Yes, the versions are incompatible for auto-sync. What are you tryingto sync which way? Many things can be done using 'export' andtry to import on the other server. It will give you a version mismatch number which can be searched and replaced in the export xml. This will work better 9.0 --> 9.5 as the other way might contain items not existing in 9.0...
I'm tried use the standard rollup sync with a scheduled task. The way is 9.5 --> Rollup Server 9.0 SP3.